Self-starting unidirectional synchronous motors are known, which have a first plurality of main poles, and a second plurality of auxiliary poles. Alternating fields are formed between the poles of the main pole group, and the poles of the auxiliary pole group of the stator, which are spatially displaced by respective electrical angles, and whose magnetic fields are shifted by respective phase angles and wherein the fields are superimposed, and together constitute a rotating field, which acts on the rotor of the synchronous motor. The stator poles of known synchronous motors have a uniform rectangular cross-section and are formed by pole arms. As two adjoining poles, within a pole group, namely poles disposed physically opposite one another and supported on different respective stator halves are designed to cooperate, there occur in end zones between the pole groups pole halves, wherein a pole half may lack a corresponding partner within the respective pole group. Hence in known synchronous motors there does not occur any uniform spatial distribution of the fields within a pole group, so that an elliptical rotating field results. When the motor is lightly loaded, it can even run backwards under those conditions. There also occur very significant fluctuations of the angular velocity of the rotor, which reduces the motor torque. The static torque, which is effective during synchronous operation of the motor, also contributes, as does the elliptical rotating field, to fluctuations of the angular velocity of the motor, and loss of total operative torque. Between the pole groups occur narrow passages for the shading rings where poles supported from the same stator portion are disposed next to one another, which arrangement reduces the effectiveness of the shading rings.
The basic operation of a synchronous motor of the type described in the present application is taught by Welch et al, U.S. Pat. No. 2,437,142 issued in 1948. Gerber teaches a small synchronous motor disclosed in Swiss patent CH 507,612 which uses only one group of main poles and one group of auxiliary poles, which poles extend approximately around respective halves of the stator periphery.
A unidirectional synchronous motor is taught by Eigeman in German patent AL 1,128,546. Eigeman teaches a synchronous motor which has a smaller number of main poles than of auxiliary poles, and where the auxiliary poles are shifted with respect to main poles of the same polarity by an angle of 360.degree.+.alpha. electrical degrees, where .alpha.=180.degree.-.psi. electrical degrees, .psi. being the phase shift between the flux of the main poles and that of the auxiliary poles of the same polarity.
Riggs, in U.S. Pat. No. 3,808,491, teaches a shaded pole unidirectional synchronous motor which employs an odd number of shaded stator poles, and an odd number of non-shaded stator poles. The number of shaded poles exceeds the number of non-shaded poles, and the shaded and non-shaded poles are arranged in groups, the separation between groups differing by 540+.alpha., or 540-.alpha. electrical degrees, where .alpha. is the shading angle. The stator poles extend from an upper cover and lower mounting plate, respectively, to form a cylindrical cage, and are designed to function with an A.C. excited toroidal coil outside the cage, and a ferrite rotor with induced poles thereon, rotatably mounted within the cage formed by the stator poles.